A fuel injection valve can employ a number of control strategies for governing the quantity of fuel that is introduced into the combustion chamber of an internal combustion engine. For example, some of the parameters that can be manipulated by commonly known control strategies are the pulse width of the injection event, fuel pressure, and the valve needle lift.
The “pulse width” of an injection event is defined herein by the time that a fuel injection valve is open to allow fuel to be injected into the combustion chamber. Assuming a constant fuel pressure and a constant valve needle lift, a longer pulse width generally results in a larger quantity of fuel being introduced into the combustion chamber.
However, fuel pressure need not be constant from one injection event to another and fuel pressure can be raised to increase the quantity of fuel that is introduced into the combustion chamber. Conversely fuel pressure can be reduced to inject a smaller quantity of fuel into the engine, for example during idle or low load conditions.
As yet another example, some types of fuel injection valves can control valve needle lift to influence the quantity of fuel that is introduced into a combustion chamber. An increase in needle lift generally corresponds to an increase in the quantity of fuel that is injected and some fuel injection valves can be controlled to hold the valve needle at an intermediate position between the closed and fully open positions to allow a flow rate that is less than a maximum flow rate. To control valve needle lift a fuel injection valve can employ mechanical devices or an actuator that is controllable to lift and hold the needle at intermediate positions between the closed and fully open positions.
European Patent Specification No. EP 0615065 B1 (“Shibata”) discloses a fuel injection valve for injecting a liquid fuel using an injection pump with a cam driven plunger that reciprocates to increase fuel pressure to actuate the fuel injection valve. The cam has a low-speed area where the fuel supply rate of the pump is low and a high-speed area where the fuel-supply rate is high so that the plunger is movable at a variable speed. The injection valve has an elongated pin formed on the nozzle needle for keeping the size of the fuel passage at the injection port substantially constant when the pin is positioned in the injection hole even when the nozzle needle moves, whereby the fuel injection mass flow rate is substantially constant until the pin is lifted out of the injection hole. Shibata discloses an apparatus and method that can be employed to shape the fuel injection mass flow rate during the course of an injection event whereby the fuel injection rate is initially low (while the pin is positioned in the injection hole), and then raised to a higher fuel injection rate (when the pin is lifted from the injection hole). However, because the injection pump is mechanically operated using a cam and plunger arrangement, the shape of the fuel injection mass flow rate is generally the same for each injection event. For each injection event the nozzle needle is continuously moving from the closed position to the fully open position and then back to the closed position, with the pin at the end of the nozzle needle providing a restriction that produces the step shaped injection pulse. Shibata does not disclose an apparatus or method for regulating fuel mass flow by actuating a valve needle that is operable to hold the valve needle at intermediate positions and a method whereby the valve needle lift is variable both during an injection event and from one injection event to another injection event. That is, Shibata does not disclose an apparatus or method that allows partial valve needle lift to an intermediate position for the duration of an injection event so that the lower mass flow rate is provided for the entire injection event, and that also allows valve lift to a fully open position for another injection event.
A difficult task for known control strategies is controlling the quantity of fuel that is injected into an engine's combustion chamber under idle or low load conditions. Under such conditions the fuel injection valve is required to inject only a small amount of fuel into the combustion chamber, and even small variations in the quantity of fuel that is injected into the combustion chamber can result in a significant variance in the injected quantity of fuel that can cause unstable operation. Under high load conditions, variations in the quantity of fuel of the same order of magnitude have less impact on engine operation because they represent a much smaller variation in the difference between the desired quantity of injected fuel versus the actual quantity of injected fuel, when this difference is considered as a percentage of the total quantity of injected fuel.
To control the quantity of fuel injected during idle and low load conditions, if the control strategy manipulates only pulse width, this strategy can result in a pulse width that is too short to provide consistent and efficient combustion. Accordingly, simply shortening pulse width at idle or low load conditions to reduce the quantity of injected fuel is not a desirable strategy.
A pulse width sufficiently long for idle or low load conditions can be achieved by reducing the fuel pressure. For liquid fuels this is a viable strategy, but it requires a system for controlling fuel pressure, adding to the cost and complexity of the fuel injection system. For example, known liquid fuel systems can reduce fuel pressure by returning a portion of the high-pressure fuel to the fuel tank. With liquid fuels, there are limitations on how low the pressure can be reduced since a minimum fuel pressure is required to atomize the fuel when it is introduced into the engine's combustion chamber. However, this approach is more difficult with a gaseous fuel. Since a gas is a compressible fluid, compared to a liquid fuel, much more gaseous fuel must be returned to the fuel tank for a comparable reduction in fuel pressure, and if the gaseous fuel tank is pressurized, there can be times when the tank pressure exceeds the fuel rail pressure, making return flow impossible. Consequently, it can be difficult to rapidly reduce the pressure of a gaseous fuel without venting some of the fuel to atmosphere, which is undesirable. Accordingly, it can be difficult to control fuel pressure to achieve the desired responsiveness for controlling the fuel injection mass flow rate during an injection event or from one injection event to the next. It can also be difficult to control fuel pressure and injection valve operation to accurately inject the exact quantity of fuel with the precision desired for each injection event, and again, only small variations in fuel quantity can cause unstable operating conditions. Therefore, controlling fuel injection pressure alone is not a desirable strategy for regulating fuel mass flow rate through a fuel injection valve.
If a fuel injection valve is operable to control valve needle lift, flow rate can be controlled to provide a sufficiently long pulse width to inject the desired quantity of fuel for an engine that is idling or operating under low load conditions. As shown in Japanese Patent Application No. 60-031204 (Japanese Patent Publication No. 61-190165), a fuel injection valve can be provided with a stopper that is movable to limit the lift of the valve needle. This type of mechanical arrangement adds considerable complexity to the fuel injection valve and, consequently, higher manufacturing costs, space requirements for installing the injection valve assembly, maintenance costs, and reliability concerns.
In another approach, fuel injection valves are known that control the quantity of injected fuel by employing variable orifice areas. That is, the injection valve can have two sets of orifices whereby the valve is operable to inject fuel through only one set of orifices when a smaller quantity of fuel is to be injected, and fuel is injected through both sets of orifices when a larger quantity of fuel is to be injected. U.S. Pat. No. 4,546,739 discloses an example of such an injection valve. Like other known mechanical solutions this arrangement adds complexity and the associated disadvantages of higher manufacturing costs, maintenance costs, and concerns for reliability.
Another type of fuel injection valve can be directly actuated by a strain-type actuator, which can be commanded to lift the valve needle to any position between its closed and open position. Co-owned U.S. Pat. Nos. 6,298,829, 6,564,777, 6,575,138 and 6,584,958, which are hereby incorporated by reference in their entirety, disclose examples of directly actuated fuel injection valves that employ a strain-type actuator. For example, if the strain-type actuator is a piezoelectric actuator, by controlling the charge applied to the actuator the valve needle lift can be commanded to the desired lift position. However, even with this approach there can be variability of fuel flow from one injection event to the next because the actual valve needle lift may not always accurately match the commanded lift. Variability in the actual valve needle lift can be caused by a number of factors, including, for example, one or more of variations in combustion chamber pressure, variations in fuel pressure, the effects of differential thermal expansion/contraction within the fuel injection valve, and component wear within the fuel injection valve. Accordingly, even with a fuel injection valve that employs an actuator that allows lift control, there can be factors that cause variability in the actual lift that can still be large enough to cause variability in the quantity of injected fuel.
Engine instability at idle and low load conditions can cause higher engine fuel consumption, exhaust emissions, noise and vibration. Accordingly, there is a need for an apparatus and method that provides a more consistent means of controlling the quantity of fuel injected during each injection event when an engine is idling or under low load conditions and that improves combustion stability under such conditions.
For compression ignition engines that burn a gaseous fuel it can be beneficial to shape the rate of fuel injection to begin an injection event with an initial low mass flow rate, followed by a higher mass flow rate until the end of the fuel injection event. An example of this is disclosed in co-owned and co-pending U.S. patent application Ser. No. 10/414,850, entitled, “Internal Combustion Engine With Injection Of Gaseous Fuel”, which is hereby incorporated by reference in its entirety. It can be difficult to operate a conventional fuel injection valve to provide the stepped flow characteristic that is needed to achieve this result. If a fuel injection valve that provides a substantially constant mass flow rate for a predetermined range of valve needle movement can be made so that this constant mass flow rate corresponds to the initial low mass flow rate for a stepped injection event, such a feature can be useful for improving injection consistency and engine performance for all operating conditions from idle through to full load.